Best thing I have seen so far is Science Team. Its strength is dependent on your skill in Science Team Leader (and possibly other things); the strength of the operations bonus it gives is based on crew (the shield recharge and shield resistance bonus is NOT dependent on crew). How often you can use it is dependent on the BO's actual skill level, which does NOT affect its strength.
Straightforward? No.
In any case, I do not recommend using skills that *only* affect other ships. You're going to need them for yourself soon enough. Fortunately, Science Team works on you as well, so it's all good.
Note: Science Team improves all four shields simultaneously. If some are full a lot of the effect is wasted, so make sure you balance your shields before using it and try to get your teammates to keep their shields balanced as much as reasonably possible.
The Engineering version is Engineering team, which restores a healthy amount of hull and also gives a damage resistance bonus to the hull and even some warp core training (which can improve power levels on non-maxed energy levels). Highly recommended as well, but doesn't affect shield damage resistance.
Supposedly Tactical team affects crew recovery and provides resistance to boarding parties. Not as useful therefore but I plan to pick it up and test it for those times that I'd really like my crew to get back in action.
So to sum up, these are the ally shield-healing bridge officer powers.
Engineering: Extend Shields Science: Science Team, Transfer Shield Strength
Each of these is available as at Ensign and Lieutenant levels normally, with Lt. Commander versions being rarer.
Which is the best? That depends a lot on your build. They are affected by different skills.
Extend Shields is improved by the various Shield-related skills in the Starship Engineering tree (Starship Shield Performance, Starship Shield Efficiency, and Starship Engineering itself, I believe).
Science Team is buffed by the Science Team Leader skill.
Transfer Shield Strength gets its bonuses from Starship Operations Training, Starship Deflectors, and Starship Deflector Field.
If you can't fit all three powers into a build (and you may not want to, anyway), pick the ones that best fits your skill point expenditure.
Note that Science Team is the only one of the three that also works on yourself.